Measurement report: Crustal materials play an increasing role in elevating particle pH: Insights from 12-year records in a typical inland city of China

Abstract. Particle acidity is a critical parameter that affects atmospheric chemistry. Concerns have been raised about the exacerbating aerosol and rainfall acidity due to China’s ongoing efforts to reduce ammonia emissions. Therefore, it is urgent to clarify the changing trends in particle pH response to air pollution control policies, especially in North China, which is significantly affected by dust aerosol. 12-years observational data in Zhengzhou reveal that the annual average PM_2.5 concentration decreased from 212 ± 102 μg/m³ in 2013 to 60 ± 41 μg/m³ in 2022, with the largest reduction in sulfate (79 %). Correspondingly, the annual particle pH increased by 0.11 units from 2013 to 2019. In addition, the elevated particle pH in 2015 and 2018 was notably influenced by the increase in TNH_x (NH₃ + NH₄⁺). Note that the crustal material concentrations and their proportions increased significantly during 2019–2022, which might be responsible for the resuspension of surrounding soil dust. Even though the TNH_x concentration was decreasing, the annual average growth rate of pH values increased to 0.21 units from 2019 to 2022. This phenomenon is not unique to Zhengzhou, as major cities in the North China Plain have also experienced a pronounced upward trend in coarse particles after 2019. Therefore, the future ammonia reduction policies in North China may not lead to a rapid increase in particle acidity buffering by the crustal materials.